1. Technical Field
The present invention relates to a solid oxide electrolyte fuel cell (SOFC) and, more specifically, to a SOFC which can be used as a cell for water electrolysis, CO.sub.2 electrolysis and other electrolyses, as well as for power generation.
2. Background Technology
A power generation film of a non-cylindrical SOFC (also called a planar or integrally layered SOFC) is composed of three layers of a solid electrolyte, an oxygen electrode and a fuel electrode. Because the respective layers are made of different materials, they have different thermal expansion coefficients at 950.degree.-1,000.degree. C. that is said to be an optimum temperature range for power generation, or different thermal contraction coefficients when they cool down after power generation. Therefore, it is difficult to sinter the three layers while forming a unified body.
Conventionally, a film of yttria-stabilized zirconia (YSZ) is used as the solid electrolyte. To obtain a fine YSZ film, firing is performed at 1,400.degree. C. or more. The above-mentioned difficulty in sintering the three layers is due to a phenomenon that when the YSZ film unified with the oxygen electrode is fired at the above temperature, an insulating material of La.sub.2 Zr.sub.2 O.sub.7 is formed at the boundary between the YSZ film and the oxygen electrode.
In view of the above, another method is taken in which a YSZ film to constitute the solid electrolyte is fired in advance at 1,400.degree. C. or more, and the oxygen electrode is formed by applying an oxygen electrode material to the YSZ film by slurry coating, spray coating or some other method and then again firing it at 1,300.degree. C. or less, which temperature does not cause formation of any insulating material.
However, the above method requires manual operations to, for instance, coat the oxygen electrode material on the YSZ film fired in advance and fire the coated oxygen electrode material. If the YSZ film is too thin, it may be broken in those manual operations.
To avoid such a problem, conventionally, the thickness of the YSZ film is increased to 100-500 .mu.m, to thereby make the YSZ film strong enough to endure at least when it is carried or where a multilayered cell is formed.
An oxygen electrode material and a fuel electrode material are applied to both surfaces of the YSZ film having such a thickness by screen printing, spray coating, brush coating, or the like at a thickness of 20-50 .mu.m, to thereby form a power generation film consisting of three layers.
Conventionally, in the above manner, the YSZ film as the solid electrolyte that constitutes the power generation film of the non-cylindrical SOFC functions in itself as a structural body.
On the other hand, yttria-stabilized zirconia (YSZ) for the YSZ film as the solid electrolyte has a conductivity of about 0.16-0.17 Scm.sup.-1, which is much smaller than 20-200 Scm.sup.-1 of the oxygen electrode material that constitutes the oxygen electrode and 500-1,000 Scm.sup.-1 of the fuel electrode material that constitutes the fuel electrode. To improve the power generation performance, it is desired to make the YSZ film as thin as possible.
However, in the conventional power generation film, the solid electrolyte (YSZ film) cannot be made thinner than a certain limit, because, as described above, the solid electrolyte itself needs to function as a structural body.